This invention relates to the field of extractive metallurgy, and more particularly to an improved process for recovery of gold from ores and gold-bearing leachates.
Conventionally, gold has been recovered from ores by leaching with alkaline cyanide solution. By reaction with cyanide ion and oxygen, the precious metal is converted to a cyanide complex (gold cyanide anion) which is taken up in the leaching solution. The precious metal is recovered from the cyanide leachate by any of a number of methods, including precipitation with a less noble metal such as zinc, direct electrowinning, ion exchange, and carbon adsorption. While widely practiced on a commercial scale, cyanide leaching suffers from well known disadvantages.
In an effort to overcome the disadvantages associated with cyanide leaching, other gold solubilizers have been proposed including halogens, halides and halide-bearing compounds. For example, Sergent et al. U.S. Pat. No. 4,637,865 describe a process for extracting a precious metal from a source material by contacting the source material with an aqueous leaching solution containing a leaching agent comprised of an N-halohydantoin compound. Leaching solutions are described containing 1,3-dibromo-5,5-dimethylhydantoin, 1-bromo-3-chloro-5,5 dimethylhydantoin and 1,3-dichloro-5,5-dimethylhydantoin.
Another commercial process for leaching of gold from a source material with an aqueous bromine leaching solution comprises contacting the source material with an aqueous bromine leaching solution having a pH of between about 2 and about 10 and containing between about 0.01% and about 20% by weight equivalent molecular bromine, between about 0.005% and about 20% by weight bromide ion, and between about 0.005% and about 30% by weight total halide ion to produce an aqueous leachate containing gold in the form a gold-bromide complex ion.
While bromine based leaching solutions offer advantages over cyanide leaching solutions with respect to extraction of the gold from the gold-bearing source material, there remains the problem of how to recover the gold dissolved in the leachate. Conventionally, the gold is recovered from the leachate by zinc or aluminum precipitation, ion exchange, carbon adsorption, or electrowinning. However, none of these methods is entirely satisfactory. Zinc and aluminum precipitation results in the formation of zinc and aluminum containing solutions which require proper disposal to avoid environmental problems. Conventional ion exchange resins are relatively expensive and must be regenerated. Gold bromide complex ion may be adsorbed on activated carbon, but the elution of gold from the activated carbon is relatively inefficient. Relative to ion exchange and zinc precipitation, electrowinning is not as efficient with respect to the recovery of gold. What is needed, therefore, is an improved process for recovering gold from leachates produced by bromine based leaching solutions.